Although a substantial number of gear oils are available in the marketplace, there exists a need for further improvements in limited slip or enhanced positraction performance.
It is known that the high pressures occurring in certain gears and bearings may cause a lubricant film to rupture so that opposing metal surfaces contact each other. This metal contact results in scuffing, seizure, excessive wear, loss of efficiency, and ultimately in the failure of the mechanism. In addition, these high pressures on the oil film effect a rise in internal heat which may be increased by any friction resulting from the metal contacts through breaks in the oil film. Consequently, mechanisms in which high mechanical pressures are likely to occur between interacting metal surfaces require lubricants that have both high lubricity and high film strength or extreme pressure properties.
Unfortunately, these two requirements are frequently antipathetic to each other. That is, an additive which has high film strength does not necessarily have good lubricity or "oiliness" and may in fact increase friction and heating in the oil film. Conversely, an additive having good lubricity or "oiliness" does not necessarily have good extreme pressure properties, and may in fact mask or interfere with the action of a separate extreme pressure additive component.
Limited slip axles or differentials are mechanisms which transmit the greater driving force to the vehicular wheel that has the better traction. Such mechanisms have a special lubrication problem peculiar unto themselves. The interplay of pressures and forces during turns under load often results in noise, often referred to as "chatter", and vibration or shudder of the vehicle. In order to function properly, limited slip axles or differentials require, among other things, lubricants that have both high lubricity and high film strength or extreme pressure properties, requirements which, as noted above, are frequently antipathetic to each other.
Prior attempts to overcome this "chatter" problem generally involved research on and utilization of friction-reducing agents or oiliness agents which were added to the base lubricant. In that approach it was necessary to avoid upsetting the balance between extreme pressure, antiwear, and rust and corrosion protection as well as the oxidative stability afforded by the additive components utilized in such lubricants. A very desirable advance in the art would be the provision of additive compositions that have the capability of providing both limited slip properties and antiwear/extreme pressure properties to a lubricating oil composition in which they are employed. Such an additive would enable the formulation of lubricants which avoid or at least minimize the "chatter" problem, contribute antiwear/extreme pressure properties, and at the same time make it possible to avoid the complications brought about by use of conventional friction-reducing additives in the lubricant. An additive possessing such multifunctionality is rarely, if ever, encountered in the art.
U.S. Pat. Nos. 3,197,405 and 3,197,496 contain extensive descriptions of phosphorus- and nitrogen-containing products formed by reaction of a hydroxy-substituted triester of a phosphorothioic acid with particular inorganic phosphorus reagents, and neutralization of the product with an amine. These products are indicated to be useful as insecticides, corrosion inhibitors, rust inhibitors, antiwear agents, and are indicated to be especially effective as corrosion inhibiting and extreme pressure additives in lubricating compositions. According to these patents, the hydroxy-substituted phosphorothioate triester may be formed in various ways including reaction of an O,O-dihydrocarbyl phosphorodithioic acid with an epoxide or glycol. The preferred epoxides are aliphatic epoxides having less than about 8 carbon atoms and styrene oxides. Other acyclic aliphatic epoxides mentioned are 1,2-octene oxide, dodecene oxide and octadecene oxide.
U.S. Pat. No. 4,435,338 describes products made by reaction of an O,O-dialkyl dithiophosphate with a difunctional, trifunctional or tetrafunctional epoxide compound.